I have disabled spell checker in System Preferences > Keyboard > Text.
Also, in TextEdit, under Edit > Grammar > Check spelling automatically.
Opening a document in TextEdit keeps re-enabling the app-level spell checker itself, again and again.
I am not using macOS in English. Running macOS Mojave 10.14.3
How can I get rid of this? I remember this was a bug in some previous Mac OS X version. It has been fixed back in the day.